Heretofore, in the field of gasoline engines, a combustion mode where an air-fuel mixture is forcibly ignited by a spark discharge from a spark plug (spark ignition (SI) combustion) has been commonly employed. Recent years, researches for applying so-called “homogeneous-charge compression ignition (HCCI) combustion” to gasoline engines in place of the SI combustion have been conducted. For example, as a document related thereto, there is JP 2007-132319A (hereinafter referred to as “Patent Document 1”). The HCCI combustion is a combustion mode in which an air-fuel mixture formed in a cylinder (combustion chamber) of a gasoline engine, is auto-ignited without relying on spark ignition, under a high-temperature/high-pressure environment created by compressing the air-fuel mixture by a piston. In the HCCI combustion, auto-ignition occurs simultaneously at many positions in the cylinder. Thus, and it is said that the HCCI combustion has a shorter combustion period and thereby achieves higher thermal efficiency, as compared with the SI combustion.
However, in the HCCI combustion, due to the shorter combustion period and the higher thermal efficiency, energy to be discharged outside as heat (exhaust loss) becomes smaller, so that an exhaust gas temperature is apt to become lower, as compared with the SI combustion. Thus, there is concern that an exhaust gas-purifying catalyst provided in an exhaust passage fails to sufficiently function.
Specifically, generally, a catalyst comprised, for example, of a three-way catalyst, is provided in an exhaust passage of a gasoline engine. As a prerequisite to allowing the catalyst to sufficiently exert purification performance, it is necessary to maintain a catalyst temperature at a value higher than a certain level. However, in cases where the gasoline engine is operated in the HCCI combustion mode where the exhaust gas temperature is likely to become lower, the catalyst temperature is also likely to become lower than a catalyst activation temperature, which causes a risk that the catalyst fails to sufficiently function.
Meanwhile, in the field of spark-ignition gasoline engines, there has been known a technique of injecting additional fuel during an expansion stroke to achieve a rise in catalyst temperature. For example, JP 2000-130212A (hereinafter referred to as “Patent Document 2”) discloses a spark-ignition gasoline engine designed to perform a so-called “stratified lean combustion mode” in which fuel (gasoline) is injected into a cylinder in a later phase of a compression stroke in such a manner as to form a rich air-fuel mixture locally around a spark plug to achieve a significantly lean air/fuel ratio in the cylinder as a whole, and then forcibly ignite the air-fuel mixture by the spark plug. The Patent Document 2 also discloses a technique of, after the fuel injection during the compression stroke, performing an additional fuel injection during a subsequent expansion stroke to induce an oxidization reaction of additionally-injected fuel in an exhaust passage, etc., so as to raise an exhaust gas temperature and thus a temperature of a catalyst (in the Patent Document 2, NOx catalyst) in the exhaust passage to facilitate performance recovery of the catalyst.
However, the above technique in the Patent Document 2, i.e., a multi-stage fuel injection for raising the exhaust gas temperature, is based on a spark-ignition lean-burn engine, but it does not assume a gasoline engine to be operated in the HCCI combustion mode. Considering that the HCCI combustion mode and the SI combustion mode are totally different in combustion mechanism, even if the multi-stage fuel injection disclosed in the Patent Document 2 is applied to an HCCI engine without modifications, auto-ignitability of an air-fuel mixture is liable to deteriorate, which causes a problem such as misfire, or an amount of fuel to be discharged outside without being combusted in a cylinder (unburned fuel) is liable to increase, which causes significant deterioration in fuel economy.